This invention relates to underwater communication systems, and more particularly to a protective cover for the sound transducer of the communication system which is usually located inside the face mask of the diver and which needs protection from moisture.
The term sound transducer, as used herein, includes not only the microphone but also the speaker even though the invention will be described with special emphasis on the microphone.
Since the face mask of a diver, even though normally dry and maintained, through breathing, at an air pressure equal to the ambient water pressure, there are times when the face mask interior is flushed with water. This may happen accidentally when the mask is ripped off or becomes defective or intentionally when the diver wants to flush the mask for cleaning the inside of the face plate. It has therefore been found necessary to protect the microphone (or speaker), located inside the mask, from the water to prevent damage or deterioration of the transducer.
Prior art moisture protective housings for transducers, and other objects including cameras, have taken on one of two different forms. One form involving total encapsulation and the other form involving a collapsible bladder. In case of the encapsulation, the housing must have rigid walls which are designed to withstand the pressure to which the diver descends. Typically, at a depth of 330 feet, the pressure would be about five atmospheres so that such a housing must be fairly sturdy to be able to withstand such pressure without collapsing. One of the disadvantages of such a rigid housing is that the quality of sound transmission across the housing wall is poor, one reason being the difference between the internal and external pressure of the housing and another reason being the barrier of the housing material itself, which is not normally transparent to sound waves. The sound from sound transducers encapsulated in a rigid housing usually suffers so much distortion and is reduced so much in fidelity to make sound communication fairly unsatisfactory.
To decrease distortion and increase fidelity, a bladder type housing is often used. Such a housing, which surrounds the microphone is usually flexible and hermetically sealed and made of a material which is far more transparent to sound waves than the rigid housing previously discussed. To prevent the bladder from being tightly compressed against the microphone, it is filled with air at an atmospheric pressure and is made of such a size that it reduces the interior volume to a size not too small to provide good sound transmission at the maximum desired depth. It has been found that, for a microphone to operate efficiently, the cavity surrounding it should not be less than a certain minimum size. And, to assure that at the maximum desired depth, the pressure on the bladder does not reduce the interior volume too much below the optimum minimum volume, the bladder must usually be fairly large. At 5 atmospheres of water pressure, which is a normal maximum diving depth, the volume of the bladder will be reduced to one-fifth of its original volume. Even though such a housing has been found fairly satisfactory as providing low distortion and high fidelity, the problem is its size at sea level pressure because, in order to maintain adequate minimum interior space around the microphone, the bladder must be larger than a face mask can conveniently accommodate.